In an LTE (including LTE-Advanced) system, a random access procedure (RA procedure, to be referred to as RA procedure hereinafter) is performed in order to synchronize timing between a user apparatus UE and a base station eNB. The RA procedure is performed, for example, in a case where the user apparatus UE initially accesses the base station eNB, or in a case where re-synchronization is performed when handover is performed, or the like. Also, as the RA procedure, there are non-contention based (contention free) RA (to be referred to as contention free RA hereinafter) which is non-contention based, and contention based RA which is contention based. For example, contention free RA is used for handover and the like, and Contention based RA is used when a user apparatus UE initially originates a call, or the like.
In the LTE system, carrier aggregation (to be referred to as CA hereinafter) is introduced which enables communication by using a plurality of component carriers (to be referred to as CC hereinafter). As shown in FIG. 1, in CA up to Rel-10 of LTE, high throughput can be realized by performing simultaneous communication using a plurality of CCs under the the same base station eNB.
On the other hand, in Rel-12, this is further extended, so that dual connectivity (dual connectivity, to be also referred to as DC hereinafter) is being studies, in which simultaneous communication is performed by using CCs under different base stations eNB so as to realize high throughput (refer to non-patent document 1). For example, in a case where all CCs cannot be accommodated in a single base station eNB, dual connectivity is necessary to realize throughput similar to that of Rel-10.
In dual connectivity, a base station that forms a Pcell is called Master-eNB (MeNB), and a base station that forms an Scell is called Secondary-eNB (SeNB). FIG. 2 shows an example of dual connectivity. In the example of FIG. 2, a base station MeNB communicates with a user apparatus UE using a CC#1 (CC of Pcell), and a base station SeNB communicates with the user apparatus UE using a CC#2 (CC of Scell) so as to realize CA.